A major goal of HIV research is the development of a new generation of antiretroviral agents that target novel stages of the viral replicative cycle. New agents are needed both to combat the growing incidence of HIV strains that are broadly resistant to existing medications and to reduce the considerable toxicities associated with current therapies. HIV entry comprises a cascade of sequential events that provide attractive targets for new therapies. The chemokine receptor CCR5 serves as a critical portal of HIV entry by acting as a fusion coreceptor in conjunction with CD4, the primary receptor for HIV. CCR5 plays a central role in virus transmission and pathogenesis, and thus CCR5-targeted therapies represent a promising new treatment modality. Small-molecule CCR5 inhibitors have been identified previously by screening for inhibition of chemokine binding, which is the natural activity of CCR5. All such molecules are potent CCR5 antagonists that may result in toxicities related to disruption of the chemokine network. Possible side-effects of chronic therapy include loss of CCR5-mediated immune function in patients who are already immunocompromised and overproduction of promiscuous chemokines. To develop inhibitors with a potentially improved therapeutic profile, we adopted the novel approach of identifying molecules that inhibit CCR5's interactions with HIV but not chemokines. The rationale for this approach is provided by the known differences in CCR5 recognition by HIV and chemokines. To achieve this goal, we screened a proprietary library of compounds for inhibition of viral entry using a novel high-throughput assay of HIV membrane fusion. Compounds were later characterized for CCR5 antagonism. This approach has succeeded in the identification of a novel series of compounds that effectively inhibit CCR5-mediated HIV entry without substantial CCR5 antagonism. This compound series thus represents a new class of HIV inhibitors. The overall goal of this Phase 1 project is to optimize this chemical class for increasing antiviral activity while maintaining lack of CCR5 antagonism. This optimization effort will employ an integrated and iterative process of medicinal and computational chemistry coupled with assessment of antiviral potency and chemokine receptor antagonism. The primary criterion for success in this Phase 1 project is the identification of one or more compounds that specifically inhibit CCR5-specific HIV-1 entry at nanomolar concentrations without CCR5 antagonism at micromolar concentrations. Such compounds may offer distinct tolerability and HIV-1 resistance profiles compared to current-generation CCR5 antagonists. Success in the project would provide inhibitors for further optimization to a clinical candidate with the potential to provide an important new form of therapy for HIV-1 infection.